CPU noise when under load

08
2014-07
  • CubeGod

    So, this question has been bugging me a lot, I used to have a few lovely little toys (well, not so much toys as a pentium pro and a really old Intel 80386DX microprocessor) What I noticed is that when the CPU's are under load they generate a most delicious noise which reminded me of when I used those computers for actual things (the 80386 had DOS 6.2 at one time and I ran a small unix server on it a few years later, the pentium ran windows 95 and then later windows 98+ and even later I somehow barely got xp running on it), I almost dare say it's nostalgia.

    Anyway, that noise made me curious, what causes it? My first hypothesis for this rumbling/humming sound is the fan, so I tried removing every last fan, the noise didn't stop, it was actually easier to hear when the whirring of the fans wasn't there, I then spent hours testing each piece of hardware and came to the conclusion that the CPU itself was making the noise and then decided that I was happy with my conclusion for then, I recently remembered it and I got curious as to what generates it.

    I don't own those machines anymore (alas I had to get rid of them due to moving) but I have observed that newer CPU's make the same noise, just a lot fainter and more subtle, so I do think it has something to do with the underlying architecture.

    Anyway, before my rambling carries off too far... What causes the noise and is there any correct terminology for it? These questions are derived from a few hours on google and getting nothing but people asking silly questions such as "Why is my hdd ticking?" or "Why is my CPU producing a high pitch noise?" (the latter is closer but not what I am looking for as the noise is low pitch and quite distinct, if you've heard it you should know what I'm talking about)

    I want to know this for a couple of reasons, curiosity is one but I also want to utilize this sound effect in a project of mine.

    Now, some of you may point out that CPU's have no movable parts and are thus incapable of generating noise, then what could possible causes be? I tried removing everything redundant from those machines and the sound prevailed, the only things I didn't remove or replace was the mobo's and the cpu's themselves (because finding such old hardware proved harder than expected. oh and this part is what really sparked my curiosity as to what was generating the sound as the fans were silent when just powered on, and the machine kept making noise without them (I ran them at short 1 minute intervals))

    • Answers
  • kworr

    Are you talking about Coil Whine? Here's a differently pitched example and strobed example.

    This can be cause by bad power supply, high load, bad grounding...

  • davidgo

    Sorry, but it could not have been the CPU. Maybe you were picking up some sound from the speaker/on-board buzzer of the device had one (possibly line noise ?).

  • Seth

    You might have already done this, but you didn't mention eliminating optical or hard drives as possible cause. They often make low pitched noises. The CPU itself shouldn't be making any noise, but there are other things on the motherboard can. David mentioned the possibility of a buzzer but capacitors can also make noise, especially towards the end of their life.


    • Related Question

    cpu - Processor noise. What generates it?
  • Stefano Borini

    So, I am probably not alone in noticing that any processor makes a high pitch buzzing sound while working, and this sound is more noticeable and varied in pitch when computations are performed. I also hear it from the GPU when doing operations like dragging a window around. It's not the fan. I also recall a group of hackers at some badass convention recording this noise while gnupg created a key, and got some information about it.

    Here it is: Acoustic cryptanalysis

    What physical phenomenon produces this sound?

    Here are some other reports

    http://www.videohelp.com/forum/archive/buziing-noise-from-processor-t369398.html

    https://www.rohitab.com/discuss/lofiversion/index.php/t11499.html


    • Related Answers
  • Fred Hamilton

    The most common audible noise coming from computers (besides fan noise, of course) is from the transformers used in the power supplies. They are switched at very high frequencies and they generate a powerful magnetic field (that's how they couple energy from one side of the transformer to the other). That powerful magnetic field is essentially a big electromagnet, so any ferromagnetic material in the vicinity will be pulled towards the transformer and pushed away from the transformer thousands of times a second. Most things are soldered down, but some things (like the windings of the transformer themselves) can have a little play, so they move back and forth at the switching frequency (or a harmonic or sub-harmonic of the switching frequency). This is the most common source of physical noise, and it can be modulated by the load on the CPU (as the current draw from the CPU changes, the magnetic field intensity and duty cycle changes). However the most common source of this kind of noise in this environment is the transformers (sometimes called inverters) used to create the high voltages for the backlight of LCD monitors and TVs.

    Since this seems to be a popular topic, I'll add a note on the other big source of noise in PCs. The noise discussed above is produced mechanically, you can hear it without any soundcards or speakers. If you're talking about noise you hear through your speakers, there's another source. CPUs and GPUs use 10s of amps of current from the power supplies, and that current varies depending on what the CPU/GPU is doing. The power supplies typically use the same ground return (usually a copper ground plane layer in the motherboard PCB) that all the other chips (including audio) use. Ohm's law says Voltage (V) = current (I) times resistance (R). An ideal ground plane (made of an ideal conductor) would be zero ohms from any point to any other point, so even 100A of current wouldn't generate a voltage (100 A * 0 ohms = 0 V). But a real-world copper ground plane has some resistance, say 0.010 ohms from one end to the other. So if the CPU current switches between 30A and 10A, the voltage across the ground plane may vary between 0.3V and 0.1V. This means that the ground the audio IC is relying on to "stay still" is actually moving up and down by 200mV. That makes the audio output of the IC jump up and down by up to 200mV (depending on what the processor is doing). Which you hear as noise.

    This is a very, very simplified example - people have written books about this topic. I'm just trying to convey the basic mechanism.

  • bobobobo

    The Toshiba m40 laptop also used to make this insanely annoying squealing noise. If you plugged in a USB key, the noise stopped.

    From here, someone writes about that squealing problem's cause:

    The problem is relating to a capacitor that is vibrating probably because there's too much power flowing through it